1. Field of the Invention
This invention concerns bed-of-nails test access jigs, and relates in particular to such jigs that are programmable, and so can be completely re-configured automatically.
2. Description of the Prior Art
Bed-of-nails test access jigs are commonly used to effect the required electrical connections between an electronic circuit constructed on a printed circuit board (PCB) and the automatic (electrical) test equipment (ATE) used to carry out the testing. PCBs are by now a well-known feature of electronic equipment, enabling the many components of a modern electronic circuit to be both supported and interconnected without using a jumble of wiring resembling a plate of spaghetti. A typical PCB comprises a board constructed from some insulating material such as a glass-fibre-reinforced epoxy resin, commonly up to about 18.times.18 in (45.times.45 cm) square, on which are formed, by a "printing" technique, a multitude of copper tracks eventually forming the electrical connections between the components to be mounted on the board (the leads of these components pass through holes in the board and are soldered to the tracks). During testing of a PCB, various points on the circuit--usually pads designed into the board's tracks--are required to be connected to the ATE. Most conveniently these connections are made via a bed-of-nails jig which contacts each of the required points by means of a spring-loaded pin. The testing may concern the "bare" board, without any components on it (looking for track continuity, and so on), or it may concern a "loaded" board, carrying the components, either in its "off", or non-powered, state, or--especially--in either of its "on", or powered, states (static, with no signal input, or dynamic with normal signal input; in this latter case the test actually examines whether the completed circuit is functioning correctly).
A bed-of-nails access jig consists of a number of fixed-position spring-loaded pins (the "nails") supported within a frame (the "bed") and connected to the ATE via individual cables. Most commonly, each particular bed-of-nails jig is specially designed and manufactured--with an appropriate number of pins suitably positioned--to carry out specific tests on a particular type of PCB, and from this it follows that to undertake a differing series of tests, and/or to conduct these tests on a different circuit, requires a different bed-of-nails jig design. Making such jigs is a well-established Art, and merely requires that some convenient framework be made which will hold the spring-loaded pins in the required positions.
The "fixed" pin bed-of-nails jig as described above has a number of disadvantages. For example, there is the need specially to design, manufacture, store and maintain one jig for each type of circuit board and/or each series of tests to be conducted. Further, the use of this particular technique is restricted in its application to some particular types of testing. Thus, when in contact with the PCB, each pin and its associated cable places a load on the circuit under test; the pin cables can, by inductive or capacitive coupling between each other, couple extraneous electrical signals to various parts of the circuit under test such that it no longer performs as it would perform if it were not connected to the access jig.
All these problems can be overcome by the use of a programmable bed-of-nails jig. Such a device would consist of a field of pins which was at least as big as the largest PCB to be tested. Individual pins would be positioned in the field such that there was one pin for every required contact position. Every pin would be automatically controlled such that it could be individually programmed to be "up" (activated, and capable of contacting the board) or "down" (de-activated, and incapable of contacting the board).
In order to meet these requirements such a device would, ideally, be capable of having a pin bed at least 18 in.times.18 in (45.times.45 cm), with the pin centres falling on a 0.025 in (0.635 mm) matrix (the requirement for having pins on such a matrix stems from the fact that PCBs are conventionally laid out on a corresponding grid). Unfortunately, within the current state of the Art such spacing cannot be achieved. Both the pin itself--which must be strong enough to withstand the force applied between it and the PCB--and the pin-activating mechanism have to be contained within the small space available, and while it might be possible to make the pins thin enough it has not so far proved possible to make their activating mechanisms so small.
One currently available system which approaches the ideal requirements consists of a field of spring-loaded pins mounted in a framework such that the pins lie on a suitable matrix (this can, in the state of the Art, be as small as 0.05 in, or 1.27 mm) above an actuating plate. On raising the actuating plate all the pins have a "raising" force applied to them via their individual springs. The required pin selection is achieved by the use of a blanking plate which is inserted above the pins (between the pin bed and the PCB under test). This plate is pre-drilled with holes such that only the required pins are allowed to rise (through the holes) into contact with the PCB, whilst the non-required pins are blocked off, and so are held in their lowered position.
An alternative approach employs the same basic mechanism but uses, instead of the apertured blanking plate, a second frame containing extension pins only in the required positions (no subsequent blanking plate is required since the effective length of the selected pins is significantly longer than that of the non-selected pins). Whilst more cumbersome than the simple blanking plate device, this extension pin technique does allow access to points on the PCB which do not line up with the pin matrix, for the required extension pins can be held in their frame at some slight angle such that, in the vertical plane, the head of the extension pin is displaced from the line of the main actuating pin.
The present-day programmable systems do provide a solution to the problems of the specially-built pin jig where the same pin configuration is required throughout the test sequence. They do not, however, provide a good solution where the pin configuration has to be changed during the testing sequence.
Alternative methods of achieving programmable access include the use of x-y coordinate probes. A probe carrier is driven automatically to the required position, the probe head being moved into contact with the PCB in a similar manner to the contact made by a bed-of-nails pin. To achieve multiple, simultaneous, probing, a number of separately-driven probe heads must be used. The disadvantages of this system lie in its speed of operation (which is too slow), the minimum spacing between two points which can be accessed simultaneously (which is too large), and the number of such probes which can be used at any one time (which is too small; normally such systems utilise either a single-probe or two-probe arrangement, although it is possible to design a machine which has up to five independently-controlled probe carriers).
One possible real solution lies in the use of individually-controlled pin control mechanisms such that from a total field of pins individual pins can be programmed to make contact with the PCB. This concept is not in itself new, but the design of a suitable pin actuating mechanism which will fit into the space available has previously not been achieved. The invention suggests an improved mechanism which will go some way to solving the problem. Specifically, the invention proposes the use of miniature clutch, or valve, mechanisms involving the utilisation of an Electro Rheological Fluid; it may accordingly be defined as a bed-of-nails device in which there are individual pin control mechanisms that utilise either the valve- or the clutch-forming abilities of an ER fluid.